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The 2-aminopyridine·2-pyridone (2AP·2PY) dimer is linked by N–H...O==C and N–H...N hydrogen bonds, providing a model for the Watson–Crick hydrogen bond configuration of the adenine·thymine and adenine·uracil nucleobase pairs. Mass-specific infrared spectra of 2AP·2PY and its seven N–H deuterated isotopomers have been measured between 2550 and 3650 cm-1 by IR laser depletion combined with UV two-color resonant two-photon ionization. The 2PY amide N–H stretch is a very intense band spread over the range 2700–3000 cm-1 due to large anharmonic couplings. It is shifted to lower frequency by 710 cm-1 or ≈20% upon H bonding to 2AP. On the 2AP moiety, the “bound” amino N–H stretch gives rise to a sharp band at 3140 cm-1, which is downshifted by 354 cm-1 or ≈10% upon H bonding to 2PY. The amino group “free” N–H stretch and the H–N–H bend overtone are sharp bands at ≈3530 cm-1 and 3320 cm-1. Ab initio structures and harmonic vibrations were calculated at the Hartree–Fock level and with the PW91 and B3LYP density functionals. The PW91/6-311++G(d,p) method provides excellent predictions for the frequencies and IR intensities of all the isotopomers.© 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Excitation of the 7-hydroxyquinoline·(NH3)3 [7HQ·(NH3)3] cluster to the S1 1ππ* state results in an O-H→NH3 hydrogen atom transfer (HAT) reaction. In order to investigate the entrance channel, the vibronic S1↔S0 spectra of the 7HQ·(NH3)3 and the d2-7DQ·(ND3)3 clusters have been studied by resonant two-photon ionization, UV-UV depletion and fluorescence techniques, and by ab initio calculations for the ground and excited states. For both isotopomers, the low-frequency part of the S1←S0 spectra is dominated by ammonia-wire deformation and stretching vibrations. Excitation of overtones or combinations of these modes above a threshold of 200–250 cm-1 for 7HQ·(NH3)3 accelerates the HAT reaction by an order of magnitude or more. The d2-7DQ·(ND3)3 cluster exhibits a more gradual threshold from 300 to 650 cm-1. For both isotopomers, intermolecular vibrational states above the threshold exhibit faster HAT rates than the intramolecular vibrations. The reactivity, isotope effects, and mode selectivity are interpreted in terms of H atom tunneling through a barrier along the O-H→NH3 coordinate. The barrier results from a conical intersection of the optically excited 1ππ* state with an optically dark 1πσ* state. Excitation of the ammonia-wire stretching modes decreases both the quinoline-O-H...NH3 distance and the energetic separation between the 1ππ* and 1πσ* states, thereby increasing the H atom tunneling rate. The intramolecular vibrations change the H bond distance and modulate the 1ππ*↔1πσ* interaction to a much smaller extent. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Mass-selected S[sub 1]↔S[sub 0] two color resonant two photon ionization (2C-R2PI) spectra, fluorescence spectra and fluorescence decay times are measured for supersonically cooled 7-hydroxyquinoline (7HQ)·(NH[sub 3])[sub n] clusters with n=4–10. For n=4, the S[sub 1]←S[sub 0] 2C-R2PI spectrum shows a 20 cm[sup -1] broad electronic origin at 27 746 cm[sup -1], followed by an intermolecular vibrational progression with band widths that increase up to ≈45 cm[sup -1]. In contrast, the 2C-R2PI spectra of the mixed 7HQ·(NH[sub 3])[sub 3]H[sub 2]O and 7HQ·(NH[sub 3])[sub 2](H[sub 2]O)[sub 2] clusters exhibit narrow bands of 1–2 cm[sup -1] width. The large band widths of 7HQ·(NH[sub 3])[sub 4] are due to a fast (k>10[sup 12] s[sup -1]) excited state process which is blocked when replacing one or more NH[sub 3] molecules by H[sub 2]O in the cluster. For the n=5–10 clusters, the 2C-R2PI spectra display two broad absorption bands peaking at 25 000 and 27 000 cm[sup -1]. The latter is characteristic of the 7-quinolinate (7Q[sup -]) anion, implying that ground state proton transfer from 7HQ to the ammonia cluster occurs for n>=5. Excitation at 27 000 cm[sup -1] leads to fluorescence from the 7Q[sup -*] anion clusters at 22 500 cm[sup -1] with τ[sub fl]=14–27 ns, and also to fluorescence emission from 7KQ[sup *] clusters at 18 370 cm[sup -1] with τ[sub fl]≈7 ns, signaling excited state anion→keto proton transfer. The 25 000 cm[sup -1] absorption band is characteristic of ground state 7KQ, implying that the n>=5 clusters also undergo ground state enol→keto tautomerization. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Mass- and isomer-selected S[SUB1]←S[SUB0] resonant two-photon ionization and S[SUB1]→S[SUB0] fluorescence spectra were measured for the 7-hydroxyquinoline·(NH[SUB3])[SUB2] [7HQ·(NH[SUB3])[SUB2]] and d[SUB2]-7-hydroxyquinoline ·(ND[SUB3])[SUB2] clusters cooled in supersonic expansions. UV/UV hole burning measurements prove that a single cluster isomer is formed. Ab initio self-consistent field and density functional calculations predict that the most stable cluster form has an ''ammonia wire'' hydrogen bonded to the -OH and N groups of the cis-7HQ rotamer. The experimental S[SUB0] and S[SUB1] frequencies are in very good agreement with the calculated normal mode frequencies for both the normal and deuterated ammonia-wire clusters. S[SUB1]←S[SUB0] excitation leads to contractions of the -O-H...N and NH[SUB3]...NH[SUB3] hydrogen bonds, as well as smaller displacements for the NH[SUB3]...N(quinoline) stretch and the in plane rotation (or bend) of the ammonia dimer relative to 7HQ. The coupling of these modes to the S[SUB1]←S[SUB0] electronic excitation indicates that hydrogen bond contractions in the excited state are important and may be prerequisite for the S[SUB1] state proton transfer processes that occur in the larger 7HQ· (NH[SUB3])[SUBn] (n≥4) clusters. The calculated electron density differences upon S[SUB1]←S[SUB0] excitation show large π-electron flows on the 7HQ moiety. However, the σ-electronic rearrangements that directly drive the hydrogen bond rearrangements are one to two orders of magnitude smaller. [ABSTRACT FROM AUTHOR]

In the Cage[1] isomer of the water hexamer, the free O-H bonds of the two end-cap water molecules can flip between "up" (u) and "down" (d) orientations, giving four conformers denoted uu, ud, du, and dd. Using the Møller-Plesset second order perturbation method and large basis sets, we calculate fully relaxed potential energy curves as a function of both u↔d torsional angles, denoted φ[SUB1], φ[SUB2]. These predict du as the lowest conformer, with uu nearly degenerate and ud and dd at 30-40 and 50- 70 cm[SUP-1] higher energy, respectively. Along φ[SUB1] the torsional barriers are about 200 cm[SUP-1], along φSUB1]. The torsional zero-point energies are high, the φ[SUB2] between 80 and 110 cm vibrational ground states are strongly delocalized and averaging of the cluster properties is important along both φ[SUB1] and φ[SUB2]. The dipole moment components vary strongly along both φ[SUB1] and φ[SUB2]: μ[SUBa] changes from +0.8 to +2.2 D, μ[SUBb] from +0.5 to +1.2 D, and μ[SUBc] from +1.4 to -0.9 D. The φ[SUB2] torsional fundamental of H[SUB2]O[SUB6] is predicted in the range 65- 72 cm[SUP1] with an intensity of ≈0.5 D[SUP2],theφ[SUP1] fundamental is in the 22- 32 cm[SUP-1] range, with an intensity of ≈0.3 D[SUP2]. Both excitations are b/c hybrid perpendicular bands with a dominant b component. The torsional overtones should be very weak. [ABSTRACT FROM AUTHOR]

The structure of the 1-naphthol (NH[SUB3])[SUB2] cluster was investigated by rotational coherence spectroscopy (RCS), mass selective one- and two-color resonant two-photon ionization (R2PI) experiments and ab initio calculations. RCS measurements yielded rotational constants of 1-naphthol (NH[SUB3])[SUB2] as A = 1197, B = 500, and C = 413 MHz, as well as those for several isotopomers. The counterpoise-corrected second-order Mφller-Plesset perturbation theory (MP2) method predicts two isomers A and B. Both structures have hydrogen bonded naphthol-OH… NH[SUB3]… (NH[SUB3] chains, with the second NH[SUB3] (bent above the proximal aromatic ring and pointing towards the p-electron system and have nearly the same binding energy. The experimental rotational constants agree better with those calculated for structure B. The B3LYP and PW91 density functional methods also predict two isomers A, B with the rotational constants of B in acceptable agreement with experiment. Based on two-color R2PI experiments using low ionization frequency to suppress cluster fragmentation, the S[SUB1]←S[SUB0] electronic origin region of the 1-naphthol (NH[SUB3])] [SUB2 - 4] cluster series as reassigned, in agreement with the work of Dedonder-Lardeux et al. [Phys. Chem. Chem. Phys. 3, 4316 (2001)]. In one-color experiments, the 1-naphthol (NH[SUB3])[SUB3] cluster fragments with nearly 100% efficiency into the 1-naphthol (NH[SUB3])[SUB2SUP+] mass channel. [ABSTRACT FROM AUTHOR]

We present an ab initio theoretical study of five low-energy isomers of the water hexamer {Chair, Cage(du)[1], Book, Prism, and Boat}, their intramolecular vibrations, binding energies De and dissociation energies D[sub 0]. Møller-Plesset second order perturbation calculations using the aug-cc-pVTZ basis set at aug-cc-pVDZ optimized geometries including vibrational zero point energy corrections predict Chair to be the most stable isomer, followed closely by Cage(du)[1] (+0.02 kcal/mol) and Book (+0.05 kcal/mol), while Prism is 0.15 kcal/mol higher. The Boat conformer is least stable at both the De and DO levels. The main focus is on the intramolecular normal modes of the five isomers. The calculated O-H stretching frequencies and intensities are compared to recent infrared spectra of water hexamer in supersonic jets, liquid-helium droplets and solid para-hydrogen matrices. The IR spectra indicate that Book and Chair are major species in the latter two environments and may also exist in supersonic jets. The (H[sub 2]O)[sub 6] gas phase interconversion equilibria are calculated and predict that the most abundant isomer is Chair below 8 K, Cage between 8-26 K, and Book above 26 K. Several of the low-frequency vibrational modes are identified as low-amplitude precursors of the Chair+-+Book ↔ Cage isomerization pathways. [ABSTRACT FROM AUTHOR]

The S[sub 1]↔S[sub 0] and S[sub 2]↔S[sub 0] vibronic spectra of the supersonically cooled 2-pyridone dimer (2PY)[sub 2] and its [sup 13]C-, d[sub 1]-, and d[sub 2]-isotopomers were investigated by two-color resonant two-photon ionization and fluorescence spectroscopies. For the C[sub 2h] symmetric (2PY)[sub 2]–h[sub 2] and (2PY)[sub 2]–d[sub 2] complexes, the 2PY moieties are equivalent and the S[sub 1]↔S[sub 0] ([sup 1]A[sub g]↔[sup 1]A[sub g]) transition is forbidden. A single H/D or [sup 12]C/[sup 13]C isotopic substitution reduces the symmetry to C[sub s], so that (2PY)[sub 2]–[sup 13]C and (2PY)[sub 2]–d[sub 1] both exhibit S[sub 1]↔S[sub 0] and S[sub 2]↔S[sub 0] transitions. The S[sub 1]/S[sub 2] state exciton splittings are 43.6 cm[sup -1] and 52.4 cm[sup -1], respectively. These are analyzed in terms of a Frenkel model and compared to calculated splittings based on ab initio monomer transition dipole moments. For (2PY)[sub 2]–d[sub 1], whose 2PY subunits are different, an excitation transfer time of 318 fs is calculated from the exciton splitting. The S[sub 1]↔S[sub 0] and S[sub 2]↔S[sub 0] spectra are analyzed and assigned. Several b[sub u] intermolecular vibrations of S[sub 1] appear via vibronic coupling to the S[sub 2](B[sub u]) state. Combination of the fluorescence data from excitation of the S[sub 1] and S[sub 2] origins and vibrational excitations of (2PY)[sub 2]–h[sub 2], (2PY)[sub 2]–d[sub 1], and (2PY)[sub 2]–d[sub 2] allows the determination of the six S[sub 0] state intermolecular vibrational frequencies.© 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Accurate O–H...O hydrogen-bond dissociation energies were measured for the supersonic-jet-cooled complexes 1-naphthol·S with S=D[sub 2]O, ethanol, oxirane, and oxetane. A mass-selective pump–dump–probe method was used, combining stimulated emission pumping with resonant two-photon ionization and ion-dip techniques. The ground-state dissociation energies D[sub 0](S[sub 0]) are 5.83±0.13 kcal/mol for d[sub 1]-1-naphthol·D[sub 2]O, 7.94±0.02 kcal/mol for 1-naphthol·ethanol, 7.71±0.14 kcal/mol for 1-naphthol·oxirane and >8.17 kcal/mol for 1-naphthol·oxetane. The D[sub 0]’s increase by 5%–7% upon excitation of 1-naphthol to the S[sub 1] state. These dissociation energies are compared to those of the analogous complexes with S=H[sub 2]O, methanol, NH[sub 3], and ND[sub 3] [Chem. Phys. Lett. 246, 291 (1996)]. The trends in D[sub 0] are compared to the electric dipole moments μ, molecular polarizabilities α¯, and gas-phase proton affinities of the H bond acceptor molecules. For the O-containing acceptors, the D[sub 0]’s correlate well with α¯, but the only good overall correlation for both O- and N-containing acceptors was found between the dissociation energies and proton affinities. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Hydrogen-bonded complexes of the photoacid 1-naphthol with NH[sub 3] and ND[sub 3] were investigated by resonant two-photon ionization, spectral hole burning, and fluorescence spectroscopies. Although the intermolecular vibrations are weak in both absorption and emission, with typical Franck–Condon factors <2% relative to the electronic origin, all six intermolecular modes were identified, namely the hydrogen bond stretch σ, the ammonia torsion τ, two in-plane wags β[sub 1] and β[sub 2], and two out-of-plane rocking motions ρ[sub 1] and ρ[sub 2]. Several ammonia torsional excitations were observed, with spacings in good agreement with the S[sub 0]- and S[sub 1] state effective torsional barriers derived by Humphrey and Pratt [J. Chem. Phys. 104, 8332 (1996)]. The β[sub 1], β[sub 2], and ρ[sub 2] vibrational excitations exhibit large (2–8 cm[sup -1]) torsional splittings, which indicate strong anharmonic coupling with the ammonia internal rotation. The observed Franck–Condon factors of the intermolecular stretching vibration imply a contraction of the O–H···N hydrogen bond by approx. 0.01 Å upon S[sub 1]←S[sub 0] excitation. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The combination of femtosecond time-resolved rotational coherence spectroscopy with high-level ab initio theory were used to obtain accurate structural information for the nonpolar antiaromatic molecule 1,3,5,7-cyclooctatetraene ([C.sub.8][H.sub.8], COT) and its perdeuterated isotopomer COT-[d.sub.8] ([C.sub.8][D.sub.8]). The effect of decreased [pi]-electron delocalization due to the twisting of adjacent C=C double bonds in COT relative to butadiene was observed for the C-C bonds but not for the C=C bonds.

The correlation of the calculated TD-B3LYP [S.sub.0] relates to [S.sub.1] vertical and adiabatic excitation energies are investigated for a series of 19 hydrogen-bonded complexes and clusters involving 7-hydroxyquinoline. The TD-B3LYP-computed adiabatic excitation energies are close to the experimentation excitation energies in absolute terms.

The development of a one-pot procedure for the synthesis of substituted 2,6-dicyanoanilines starting from readily available ynones and malononitrile is reported. The synthesis provides a new one-pot pathway for regioselective formation of polysubstituted benzenes.

The intermolecular binding energies [D.sub.e] of the ten doubly hydrogen bonded [uracil.sub.2] isomers are investigated by the correlated resolution of identity MP2 (RMIP2) method. All structures are optimized at the RIMP2/aug-cc-pVTZ level.

The five singly and doubly hydrogen bonded dimers of formamide are calculated at the correlated level by using resolution of identity Moller-Plesset second-order perturbation theory (RIMP2) and the coupled cluster with singles, doubles, and perturbative triples [CCSD(T)] method. A comparison with the N...O hydrogen bond distances in crystalline formamide has shown that a reversal of H-bond lengths occurs with respect to the gas-phase dimer, where the slightly longer gas-phase FA4 hydrogen bond becomes shorter and the shorter gas-phase FA1 H-bond becomes longer in the crystal.

The sugar-edge, Watson-Crick and wobble hydrogen-bonding sites of uracil and 1-,3-, and 5-methyluracilusing 2-pyridone as a probe chromophore are probed. The Watson-Crick wobble isomers are thereby determined to be several kcal/mol less strongly bound than the sugar-edge isomers.

The structure, rotational constants, vibrational frequencies, and binding and dissociation energies of (2-pyridone)(sub 2) are calculated at the correlated level using second order Moller-Plesset perturbation theory with medium to very large basis sets. It is implied that (2PY)(sub 2) is the most strongly bound doubly hydrogen-bonded dimer so far.

Investigates the coupling of a Jahn-Teller pseudorotation in a 9-hydroxytriptycene molecule. Interpretation of the triptycene spectra; Quantification of potential energy surfaces of vibrational modes; Interaction between Jahn-Teller and torsional potentials.

Presents the calculation of a four-dimensional torsional potential for the cyclic water tetramer using high-level ab initio methods. Use of the Gaussian basis set; Calculations of ab initio in the asymmetric unit of the torsional space; Representation of the symmetry relationships between sets of degenerate stationary points and interconversion pathways.

The intermolecular potential energy surface and the torsional-puckering (TP) motion of the cyclic homodromic water pentamer were theoretically investigated at the level of second-order MoilerPlesset perturbation theory using double- and triple-zeta correlation-consistent basis sets including diffuse functions. Extensive searches yielded three cyclic homodromic stationary points: (i) the puckered global minimum structure GS, (ii) a torsional-puckering saddle point, 62 cm[SUP-1] higher, denoted TS1, and (iii) a planar C[SUB5h] symmetric structure (Hessian index 7), 800 cm[SUP-1] higher in energy. Harmonic vibrational frequencies and normal modes were calculated for all stationary points. Based on the energies and second derivatives of the GS and TS 1 structures a cyclic minimum energy path for the large-amplitude TP motion with ten permutationally equivalent minima was constructed. The ground and all excited states are delocalized by TP pseudorotation, giving vibrationally averaged C[SUB5h] symmetry. The first pseudorotational TP level lies at ≈1 cm[SUP-1] (≈0.6 cm[SUP-1]) and the ten lowest TP states within a band of ≈20 cm[SUP-1] (8 cm[SUP-1]) for (H[SUP2]O)[SUB5] [(D[SUB2]O)[SUB5]]. The corresponding TP dipole moment functions were also calculated, and intensities for the far-infrared torsional-puckering transitions of (H[SUB2]O)[SUB5] and (D[SUB2]O)[SUB5] evaluated up to 300 cm[SUP-1]. [ABSTRACT FROM AUTHOR]

We report a combined experimental and theoretical study of the van der Waals isomers and intermolecular vibrations of the 2,3-dimethylnaphthalene·Ne complex in the S1 electronic state. The two-color resonant two-photon ionization spectrum exhibits eight bands within ≊40 cm-1 of the electronic origin. Theoretical considerations in combination with hole-burning spectroscopic measurements show that the transition closest to the electronic origin (at 000+5 cm-1) arises from an isomer which is different from that responsible for the other seven bands in the spectrum. The latter involve excitations of the intermolecular vibrations of the main isomer of 2,3-dimethylnaphthalene·Ne. Accurate three-dimensional quantum calculations of the van der Waals vibrational levels of the complex were performed using a discrete variable representation method. Combination of theory and experiment led to a complete assignment as well as to a quantitative theoretical reproduction of the experimental intermolecular vibrational level structure, and a parametrization of the intermolecular potential energy surface, modeled as sum of atom–atom Lennard-Jones pair potentials. This potential surface exhibits a global minimum above (and below) the aromatic ring plane of 2,3-dimethylnaphthalene and a shallower local minimum at C2v geometry, on the C2 axis of the molecule, adjacent to the two methyl groups. The main and minor isomers identified experimentally are associated with the global and the local minimum, respectively. The quantum calculations were extended to ≊1000 van der Waals vibrational states, i.e., to energies up to 78% of D0. These include levels localized either in the global or local minima, as well as highly excited vibrational states delocalized over all three potential minima, providing comprehensive insight into the quantum dynamics of the high-lying van der Waals states of an atom–large aromatic molecule complex. © 1995 American... [ABSTRACT FROM AUTHOR]

We report a detailed experimental and theoretical study of the intermolecular vibrational levels of the 1,2-dimethylnaphthalene·Ar van der Waals complex in the S1 electronic state. Due to the low symmetry of the aromatic molecular substrate, excitations in all three van der Waals modes (two in-plane, one out-of-plane) are allowed in the electronic spectrum, leading to the most complete data set of intermolecular vibrational transitions measured so far for an atom-large molecule complex. Two-color resonant two-photon ionization spectra reveal twelve bands within ≊95 cm-1 of the electronic origin. Accurate quantum three-dimensional calculations of the van der Waals vibrational levels were performed, using a method based on the 3D discrete variable representation. Combination of theory and experiment allowed a complete and quantitative interpretation of the level structure, as well as an accurate parametrization of the intermolecular potential energy surface (PES), modeled as sum of atom–atom Lennard-Jones pair potentials. The parametrization was partly transferable from other aromatic molecules, but methyl group parameters had to be adapted. Quantum number assignments of the vibrational states were possible for the low-lying levels, but difficult or impossible for states at higher energies, due to strong mode coupling, mainly between the two in-plane modes. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A spectroscopic study of supersonic jet-cooled catechol (1,2-dihydroxybenzene) and its d1- and d2-isotopomers, deuterated at the hydroxy groups, was performed by resonant two-photon ionization (R2PI) and fluorescence emission techniques, and supplemented by molecular-beam hole-burning experiments. The latter prove that one single rotamer of catechol is dominant under molecular beam conditions. The complicated vibrational structure in the S0→S1 spectrum from the 000 band to 400 cm-1 above is not due to three different rotamers, as previously thought, but is due to the excitation of a vibrational progression associated mainly with the torsion of the hydroxy groups. The torsional bands are very prominent in the R2PI spectra, but are weak in the emission spectra. Detailed analysis of the torsional bands was based on a fit to the S1 and S0 state frequencies and the Franck–Condon factors in absorption and emission, using a double-minimum potential for the S1 state and a harmonic potential for the S0 state. In the S1 state one of the two –O–H torsional mode frequencies is lowered from τ2≊250 to ≊50 cm-1, and the molecule is only quasiplanar with respect to the –O–H torsional coordinates. [ABSTRACT FROM AUTHOR]

A combined experimental and theoretical study of the intermolecular vibrations of the o-xylene·Ar van der Waals complex is reported for both the S0 and S1 electronic states. Two-color resonant two-photon ionization and fluorescence emission spectra of the vdW mode region of supersonic jet-cooled o-xylene·Ar exhibit five bands within 70 cm-1 of the electronic origin, which arise from low-frequency large-amplitude intermolecular vibrations. Accurate quantum 3D calculations of vdW vibrational levels were performed, based on the 3D discrete variable representation. Apart from the restriction to the J=0 state the calculated eigenstates are exact for the intermolecular potential energy surface (PES) employed. The PES is represented as a sum of Lennard-Jones (LJ) pair potentials, and the direct comparison between theory and experiment enabled calibration of the LJ parameters. Very good agreement was achieved for both the S0 and S1 states of o-xylene·Ar. The quantum 3D calculations provide a quantitative description of the vdW level structure up to ≊70 cm-1 above the vdW ground state. The low-energy eigenfunctions have nodal patterns analogous to the 2,3-dimethylnaphthalene·Ar complex. However, in the energy range 40–60 cm-1 the vdW mode eigenfunctions change over to 2D radial-oscillator-type wave functions, similar to those of benzene·Ar, but switch back to Cartesian type above 60 cm-1. The S1 state vdW levels of 2,3-dimethylnaphthalene [M. Mandziuk, Z. Bacˇic, T. Droz, and S. Leutwyler, J. Chem. Phys. 100, 52 (1994)] were recalculated with the present parameters, and the agreement between experimental and calculated frequencies is improved. [ABSTRACT FROM AUTHOR]

The effect of rare gas complexation on the electronically excited S1(E’) state of triptycene (T), which is Jahn–Teller distorted, was investigated by two-color resonant two-photon ionization (2C-R2PI) spectroscopy of the supersonically cooled van der Waals complexes triptycene·Nen, n=1–3. These complexes afford unique possibilities to study the effects of weak intermolecular interactions on the intramolecular Jahn–Teller coupling. Since the atoms are adsorbed at high-symmetry positions, the system symmetry is lowered from D3h(n=0) to C2v for n=1 and 2, but reverts to D3h for n=3. A Jahn–Teller (A1+E)xe coupling model including a uniaxial external strain component was applied successfully to calculate the S1 state levels and S1←S0 electronic spectra of all three complexes. The spectrum of T·Ne3 was fully interpreted without inclusion of strain, implying a highly symmetric D3h structure in which each of the three V-shaped compartments of triptycene is occupied by a single Ne atom. In contrast, the vibronic spectra of T·Ne and T·Ne2 were fitted with a considerable uniaxial strain of -19.73 and 19.07 cm-1, respectively, confirming both the predicted C2v geometry of the complexes as well as the equal magnitude, but opposite sign of the intramolecular distortion induced by one or two Ne atoms. These distortions correspond to a small change of the angle of the V-shaped compartments by ∼0.8°. The spectra of T·Ne and T·Ne2 are much more complex compared to triptycene and T·Ne3 due to the splitting of the E vibronic levels in C2v symmetry, and the appearance of additional transitions to levels that are weak or symmetry forbidden in bare triptycene. The calculated Jahn–Teller potential energy surfaces and vibronic wave functions are discussed and classified in terms of their symmetry and localization/delocalization properties. [ABSTRACT FROM AUTHOR]

The vibronic structure in the S1(E)←S0(A1) resonant two-photon ionization (R2PI) spectrum of supersonically cooled 9-fluorotriptycene is assigned using three different Jahn–Teller (JT) model Hamiltonians for the excited 1E state—Exe, (A+E)xe, and (A+E)x(a2+e). The basic Exe interpretation is satisfactory. However, the fitted vibronic band frequencies and intensities are improved by including coupling to a second excited state 1A1 in an exciton model. Some further observed absorption bands are only assignable by invoking a molecular Barnett effect (momentum coupling to an a2 vibration). The measured fluorescence emission spectra from different S1 vibronic levels are quantitatively reproduced within all three coupling schemes by the parameters fitted to the R2PI spectrum. Results are compared to previous calculations on unsubstituted triptycene. The JT stabilization energy is decreased by ∼10% upon fluoro bridgehead substitution, which is rationalized by the electron-withdrawing effect of the F atom. For the same reason, the exciton splitting between the S1 and the S2 states, as calculated in the (A+E)xe model, is reduced relative to triptycene. The ground state vibrational frequencies in the range 0–700 cm-1 are calculated using the semiempirical MOPAC 6.0/AM1 method and compared with the measured S0 frequencies, as well as those of triptycene. A 273 cm-1 degenerate C–F bending mode predicted by the AM1 calculation may explain several unassigned features in the higher-energy (200–360 cm-1) part of the R2PI spectrum, and may represent an example of e+e multimode coupling within a degenerate electronic state. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations were performed for van der Waals clusters naphthalene·Arn, n=1 to 4. For all isomers and conformers of these clusters, dynamical quantities such as velocity autocorrelation functions, vibrational power spectra, and semiclassical electronic absorption spectra were calculated over a wide energy range, and averaged over a canonical distribution at temperatures in the range T=5 to 30 K. Electronic absorption spectra were calculated for the origin bands according to the semiclassical method [L. E. Fried and S. Mukamel, J. Chem. Phys. 96, 116 (1992)] and are compared with the corresponding experimental naphthalene·Arn R2PI spectra [T. Troxler and S. Leutwyler, J. Chem. Phys. 95, 4010 (1991)]. The appearance of distinct absorption bands due to specific isomers for a given cluster size, as observed experimentally, is well reproduced by the simulations. Comparison of calculated electronic shifts for different isomers allows clear assignments in the experimental spectra. Increasing the simulation temperature to T=15–25 K is accompanied by band broadening and the appearance of sidebands towards the blue. Especially strong sidebands appear for naph·Ar2 and all clusters containing the Ar2 subunit, due to large-amplitude surface rotation/translation of the argon dimer on the naphthalene surface, in agreement with experiment. For clusters containing the n=3 and n=4 subunits the spectral broadening is smaller. For the n=4 (4+0)-isomer, the calculated band shape increases less than for the other n=4 isomers, mainly due to the motional narrowing effect of cluster fluxionality. Above 25 K, isomerization between different possible topological structures also occurs by side-crossing motion of one or several argon atoms. [ABSTRACT FROM AUTHOR]

Van der Waals (vdW) clusters of naphthalene with argon were synthesized in supersonic beam expansions. Mass-selective absorption spectroscopy was carried out by using fragmentation-free two-color resonant two-photon ionization with mass-spectrometric detection. Electronic spectra of naph·Arn (n≤6) were recorded at the vicinity of the naphthalene S1←S0 electronic origin (32 020 cm-1 ), and corresponding spectra for the 810 transition up to n=30. For n=3, the spectra due to two different isomers could be separated. Clear evidence for the existence of different cluster isomers was also found for n=6, 8, 9, 12–14, and 25–28. In comparison with most other solvent clusters M·Arn (M is the aromatic molecule), the spectral red shifts of 000 and vibronic bands of the naph·Arn clusters are very small. Analysis of the size dependence of the electronic spectral shifts indicates that the stepwise solvation of the naphthalene molecule proceeds predominantly on one side of the molecule. Two semicyclic trends in the spectral shifts are interpreted as successive wetting-nonwetting transitions which occur with increasing solvent cluster size. In the proposed mechanism for these transitions, part of the innermost solvent layer ‘‘unwets’’ the substrate, and moves to the second or third layer, thereby forming a nonwetting dropletlike cluster on one side of the naphthalene substrate. [ABSTRACT FROM AUTHOR]

Excited-state proton transfer from 1-naphthol to water was studied as a function of solvent system size, from supersonically cooled neutral clusters, 1-naphthol·(H2O)n, n=1–50, to bulk ice and water. Occurrence or nonoccurrence of proton transfer was detected and studied using cluster-size-specific laser-spectroscopic techniques: resonant two-photon ionization (R2PI) and laser-induced fluorescence emission. Depending on cluster size or solution phase, three qualitatively different types of excited-state behavior were observed: (1) For small clusters, n≤7, both the R2PI and fluorescence spectra of the clusters were similar in nature to the spectra of bare 1-naphthol; (2) the medium-size clusters (n=8–20) show incremental spectral shifts which indicate successive stages of molecular solvation, and the spectra approach that of 1-naphthol in bulk ice at n≊20; (3) the fluorescence spectra for large clusters, n≥20, show increasing emission intensity below 25 000 cm-1, characteristic of the emission of the excited-state 1-naphtholate anion. Excited-state proton transfer appears to occur in the largest observed clusters (n≥30), yet the fluorescence spectra do not converge fully to that of 1-naphtholate anion in bulk water. These three behaviors are discussed in terms of a model based on three distinct excited states connected by two electronic and geometric rearrangement processes. The model accounts in a unified way for the complete range of aqueous solvation behavior observed here as well as in many other solvent systems studied previously. The extent of proton transfer reaction is largely solvent controlled, the major determinants being the proton affinity of the solvent or solvent cluster, and its ability to resolvate the nascent ion pair on a subnanosecond time scale. In bulk ice, the slow solvent relaxation results in complete absence of excited-state proton transfer from both 1- and 2-naphthol. [ABSTRACT FROM AUTHOR]

Efficient excited-state proton transfer in neutral acid–base clusters α-naphthol·Bn has been detected and studied by a combination of laser spectroscopic techniques (resonant two-photon ionization, fluorescence excitation, and emission spectroscopy). S1 state proton transfer was observed for B=NH3 and n≥4, as evidenced by several criteria: (a) large red shift and substantial broadening of the R2PI spectra of the n≥4 clusters relative to those of the bare α-naphthol and smaller clusters; (b) very large Stokes shift (∼8000 cm-1) of the emission spectra of the n≥4 clusters; (c) complete broadening of the fluorescence emission band for the n≥4 clusters; and (d) a striking similarity of the emission band position and width of the latter spectra to the emission spectrum of the α-naphtholate anion in basic aqueous solution. No proton-transfer reaction was observed for small solvent clusters with B=NH3 and n≤3, nor for any of the pair complexes studied, which involve a single base partner [B=triethylamine, 3-dimethylamino-1-aminopropane, 1,4-bis(dimethylamino)butane] which we have studied so far. This behavior illustrates the difficulty of achieving charge separation in neutral gas-phase complexes or clusters. A critical gas-phase proton affinity PAcrit =248±3 kcal/mol was determined for proton transfer to take place in the α-naphthol·Bn (or base B) system. [ABSTRACT FROM AUTHOR]

The structures, binding energies, and vibrational frequencies of fully optimized water clusters (H2O)n, n=1– 4, were computed with ab initio molecular orbital theory at the SCF level using different basis sets. The SCF procedure allows satisfactory predictions for these properties compared with experimental results. For quantitative predictions of binding energies and geometrical parameters, a basis set including polarization functions is needed. With respect to intramolecular vibrational frequencies and frequency shifts, however, the split valence basis set 4-31G leads in all cases to the best rationalization of the available experimental data. Analysis of intramolecular force constants, frequencies, and eigenvectors for n=2 to 4 shows that (i) a transition from highly localized (n=2) to highly delocalized (n=4) vibrational modes takes place; (ii) the delocalized O–H vibrations of cyclic (H2O)n clusters (n≥3) can be described as longitudinal/transverse optical phonons; (iii) internal force constants for the hydrogen bonding O–H stretch, k(O–Hb), decrease strongly with size, leading to a decrease in the νbridge O–H frequencies; a similar decrease is found for the intramolecular bending force constant k(Θ); (iv) force constants for the free O–H stretching motions k(O–Hf) stay nearly constant, as do the corresponding vibrational frequencies; (v) intermolecular stretch–stretch couplings increase substantially with n and dominate over intramolecular stretch–stretch couplings for n≥3; similarly, intermolecular bend–libration couplings are very important. [ABSTRACT FROM AUTHOR]

Studies the intermolecular vibrations and van der Waals isomerism of the 2,3-dimethylnaphthalene...He van der Waals complex. Potential energy surface; Quantum calculations of the van der Waals vibrational levels; Reproduction of observed bands as intermolecular vibration excitations; Delocalization of van der Waals over the potential surface.

Presents information on a study which obtained diamond powders by carbon dioxide-laser-induced decomposition of carbon hydrogen at low pressures and temperatures. Methodology of the study; Results; Conclusions.

The optical absorption spectra of the B 2Π3/2 ← X 2Π3/2 transitions of chloro-, bromo-, and iodocyanide cations in neon matrices at 4.5 K have been obtained and vibrationally analyzed. The origin band of the A 2Σ+ ← X 2Π3/2 transition of ICN+ is also observed as well as the absorption bands of the CN radical in the spectra of ClCN+ and BrCN+. These features and the vibrational frequency data are discussed. [ABSTRACT FROM AUTHOR]

A combined experimental and theoretical study of the 2-naphthol·H2O/D2O system was performed. Two different rotamers of 2-naphthol (2-hydroxynaphthalene, 2HN) exist with the O–H bond in cis- and trans-position relative to the naphthalene frame. Using Hartree–Fock (HF) calculations with the 6-31G(d,p) basis set, fully energy-minimized geometries were computed for both cis- and trans-2HN·H2O of (a) the equilibrium structures with trans-linear H-bond arrangement and Cs symmetry and (b) the lowest-energy transition states for H atom exchange on the H2O subunit, which have a nonplanar C1 symmetry. Both equilibrium and transition state structures are similar to the corresponding phenol·H2O geometries. The H-bond stabilization energies with zero point energy corrections included are ≊5.7 kcal/mol for both rotamers, ≊2.3 kcal/mol stronger than for the water dimer, and correspond closely to the binding energy calculated for phenol·H2O at the same level of theory. Extension of the aromatic π-system therefore hardly affects the H-bonding conditions. The barrier height to internal rotation around the H-bond only amounts to 0.5 kcal/mol. Harmonic vibrational analysis was carried out at these stationary points on the HF/6-31G(d,p) potential energy surface with focus on the six intermolecular modes. The potential energy distributions and M-matrices reflect considerable mode scrambling for the deuterated isotopomers. For the a’ intermolecular modes anharmonic corrections to the harmonic frequencies were evaluated. The β2 wag mode shows the largest anharmonic contributions. For the torsional mode τ (H2O H-atom exchange coordinate) the vibrational level structure in an appropriate periodic potential was calculated. On the experimental side resonant-two-photon ionization and dispersed fluorescence emission spectra of 2HN·H2O and d-2HN·D2O were measured. A detailed assignment of the bands in... [ABSTRACT FROM AUTHOR]

Extensive ab initio calculations of the phenol·H2O complex were performed at the Hartree–Fock level, using the 6-31G(d,p) and 6-311++G(d,p) basis sets. Fully energy-minimized geometries were obtained for (a) the equilibrium structure, which has a translinear H bond and the H2O plane orthogonal to the phenol plane, similar to (H2O)2; (b) the lowest-energy transition state structure, which is nonplanar (C1 symmetry) and has the H2O moiety rotated by ±90°. The calculated MP2/6-311G++(d,p) binding energy including basis set superposition error corrections is 6.08 kcal/mol; the barrier for internal rotation around the H bond is only 0.4 kcal/mol. Intra- and intermolecular harmonic vibrational frequencies were calculated for a number of different isotopomers of phenol·H2O. Anharmonic intermolecular vibrational frequencies were computed for several intermolecular vibrations; anharmonic corrections are very large for the β2 intermolecular wag. Furthermore, the H2O torsion τ around the H-bond axis, and the β2 mode are strongly anharmonically coupled, and a two-dimensional τ/β2 potential energy surface was explored. The role of tunneling splitting due to the torsional mode is discussed and tunnel splittings are estimated for the calculated range of barriers. The theoretical studies were complemented by a detailed spectroscopic study of h-phenol·H2O and d-phenol·D2O employing two-color resonance-two-photon ionization and dispersed fluorescence emission techniques, which extends earlier spectroscopic studies of this system. The β1 and β2 wags of both isotopomers in the S0 and S1 electronic states are newly assigned, as well as several other weaker transitions. Tunneling splittings due to the torsional mode may be important in the S0 state in conjunction with the excitation of the intermolecular σ and β2 modes. [ABSTRACT FROM AUTHOR]

The first singlet electronic excited state of triptycene, as measured by resonant two-photon ionization in a supersonically cooled beam, has been found to be a textbook example of the E’xe’ Jahn–Teller effect. Here it is shown that this E’xe’ vibronic coupling can be profitably viewed as a subset of a (A’1+E’)x(a’2+e’) vibronic coupling scheme which results from a simple trimer model. The enlarged coupling scheme has a simple physical interpretation where the wagging coordinates of the benzene subunits are strongly coupled to their excimer formation. The previously obtained parameters, in which there is a large reduction between the ground and excited electronic state frequencies of the lowest frequency e’ mode as well as an unusually large second-order vibronic coupling constant, are shown to arise naturally from a trimer viewpoint. Features of the spectra have been found which are attributed to the involvement of an a’2 vibration which couples through nonzero momentum rather than coordinate matrix elements. A coupling of this type has been termed the ‘‘molecular Barnett effect’’ and has been predicted to appear in Jahn–Teller systems when certain conditions are fulfilled. This effect has not been previously observed in molecular spectroscopy. It indicates that crude adiabatic basis functions are inadequate to describe the electronic states of the present system. [ABSTRACT FROM AUTHOR]

In this paper we report on the electronic spectroscopy of mass-resolved tetracene·Arn (n=1–26) and tetracene·Krn (n=1–14) heteroclusters, utilizing two-photon, two-color near-threshold ionization in conjunction with mass-spectrometric detection. The spectra of the S0 → S1 transition and the ionization threshold of these heteroclusters were monitored. The structured spectral features of the S0 → S1 transition of small- and medium-sized (n=1–8) heteroclusters were attributed to the electronic origins of structural isomers and to their intermolecular vibrations. The S0 → S1 spectra of large (n≥9) heteroclusters are broad and were assigned to inhomogeneous broadening due to the coexistence of isomers, with the spectral feature(s) of each distinct isomer being homogeneously broadened. Isomer-specific inhomogeneous line broadening was interrogated by the observation of isomer-specific ionization potentials for medium-sized (n=6–7) heteroclusters and of the dependence of the relative intensities of the spectral features on the conditions of the supersonic expansion.The ionization thresholds of the tetracene·An (A=Ar,Kr) reveal a linear (or superlinear) n dependence, being qualitatively different from the sublinear n dependence of the spectral shifts. These different patterns of the size dependence can be traced to the different intermolecular interactions which govern excitation and ionization and to the difference in the charge distribution in S0 and in the positive ion. The experimental spectroscopic data for the spectral shifts and the spectral linewidths were simulated in terms of the first and second moments of the classical line shape, which were obtained from Monte Carlo (MC) constant temperature simulations, in conjunction with a two-parameter fit of the excited-state tetracene–rare-gas potential. The Monte Carlo simulations of the structural fluctuation parameters identified... [ABSTRACT FROM AUTHOR]